The host will have no darkness problems so this will only affect the client. Unfortunately there was no way we could come around this problem. Now the second problem, once your friend connects he or she may notice that the map appears to be very very dark or even pitchblack so don't be surprised if your friend appears to be looking for something with the flashlight. Keep in mind my test was done over the internet I have a 1.5mb connection and my friend has 768kpbs. Right with that out of the way you should be able to connect to each other's games (Hopefully). You may also want to change game option to no frag limit and no time limit to avoid problems once you're ingame.

When your friend then enters the game make sure his status always remains 'not ready'. When you host your own game game as soon as you enter make sure your status is ready.

A friend and I eventually came around this problem quite easily though I'm not sure whether it is what causes it. Many people have complained about the game crashing as soon as a client connects. Ok well i'm sure some of you have if not all heard about a doom 3 coop mod and have had probs connecting to or hosting your own coop games.

Doom 3 coop pc